Semiconductor-on-insulator SRAM configured using partially-depleted and fully-depleted transistors

ABSTRACT

A static memory element includes a first inverter having an input coupled to a left bit node and an output coupled to a right bit node. A second inverter has an input coupled to the right bit node and an output coupled to the left right bit node. A first fully depleted semiconductor-on-insulator transistor has a drain coupled to the left bit node, and a second fully depleted semiconductor-on-insulator transistor has a drain coupled to the right bit node.

This application is a division of U.S. application Ser. No. 10/700,869, filed Nov. 4, 2003, now U.S. Pat. No. 7,301,206 entitled “Semiconductor-on-Insulator SRAM Configured Using Partially-Depleted and Fully-Depleted Transistors” which application is hereby incorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following U.S. patents and/or commonly assigned patent applications are hereby incorporated herein by reference:

Attorney Patent No. Filing Date Issue Date Docket No. 6,855,990 B2 Nov. 26, 2002 Feb. 15, 2005 TSMC2002-0895 6,720,619 B1 Dec. 13, 2002 Apr. 13, 2004 TSMC2002-0979 6,867,433 B2 Apr. 30, 2003 Mar. 15, 2005 TSMC2003-0111

TECHNICAL FIELD

The present invention relates generally to semiconductor devices and, more particularly, the preferred embodiment relates to a semiconductor-on-insulator SRAM configured using partially-depleted and fully-depleted transistors.

BACKGROUND

The desire for higher performance circuits has driven the development of high-speed sub-100 nanometer (nm) silicon-on-insulator (SOI) complementary metal-oxide-semiconductor (CMOS) technology. In SOI technology, metal-oxide semiconductor field-effect transistors (MOSFETs) are formed on a thin layer of silicon overlying a layer of insulating material such as silicon oxide. Devices formed on SOI offer many advantages over their bulk counterparts, including reduced junction capacitance, absence of reverse body effect, soft-error immunity, full dielectric isolation, and absence of latch-up. SOI technology therefore enables higher speed performance, higher packing density, and reduced power consumption.

The most commonly used and most manufacturable SOI technology is partially-depleted SOI (PD-SOI) technology where transistors fabricated have a partially-depleted body region, that is, a PD-SOI transistor is one in which the body thickness is thicker than the maximum depletion layer width W_(d,max), so that a region of the body is undepleted. The undepleted body of the PD-SOI transistor is not tied to any voltage and is commonly described as being a floating body region.

Although PD-SOI transistors have the merit of being highly manufacturable, significant design burden is faced by its users because of floating body effects. In PD-SOI transistors, charge carriers generated by impact ionization near the drain/source region accumulate near the source/drain region of the transistor. When sufficient carriers accumulate in the floating body, which is formed right below the channel region, the body potential is effectively altered.

Floating body effects occur in PD-SOI devices because of charge build-up in the floating body region. This results in kinks in the device current-voltage (I-V) curves, thereby degrading the electrical performance of the circuit. In general, the body potential of a PD-SOI device may vary during static, dynamic, or transient device operation, and is a function of many factors like temperature, voltage, circuit topology, and switching history. Due to the fact that the body potential of the PD-SOI transistor depends on switching history, the device characteristics therefore depend on switching history, giving rise to a phenomenon known as history effect. Therefore, circuit design using PD-SOI transistors is not straightforward, and there is a significant barrier for the adoption of PD-SOI technology or the migration from bulk-Si design to PD-SOI design.

SUMMARY OF THE INVENTION

The preferred embodiment of the present invention relates generally to the fabrication of semiconductor devices. More particularly, the preferred embodiment of the present invention relates to semiconductor-on-insulator static random-access memory (SRAM) cells incorporating partially-depleted, fully-depleted transistors.

In a first embodiment, a static memory element includes a first inverter having an input coupled to a left bit node and an output coupled to a right bit node. A second inverter has an input coupled to the right bit node and an output coupled to the left right bit node. A first fully depleted semiconductor-on-insulator transistor has a drain coupled to the left bit node, and a second fully depleted semiconductor-on-insulator transistor has a drain coupled to the right bit node.

As an example, one embodiment includes a memory array formed from a plurality of SRAM cells arranged in an n by m matrix. The array includes a number of complementary bit line pairs, each including a left bit line and a right bit line. Crossing the bit line pairs is a number of word lines. An SRAM cell is located at the intersection of each word line and bit line pair. Each SRAM cell includes first and second inverters including a p-channel semiconductor-on-insulator pull-up transistor and an n-channel semiconductor-on-insulator pull-down transistor. Each cell also includes a first and second fully depleted semiconductor-on-insulator transistor that serve as access transistors.

In a more specific embodiment, a CMOS SRAM cell includes a pair of p-channel fully depleted transistors (P-FDFETs), each having a common source coupled to a voltage potential and a gate coupled to a drain of the other P-FDFET. A pair of n-channel partially depleted transistors (N-PDFETs) each have a drain connected to the drain of the respective P-FDFET of the pair of P-FDFETs, a common source connected to ground, and a gate connected to the drain of an opposite P-FDFET of the pair of P-FDFETs. A pair of n-channel pass-gate transistors each has the drain respectively connected to a connection linking the respective drain of the N-PDFET of the pair of N-PDFETs to the drain of the P-FDFET of the pair of P-FDFETs. A complementary bit line is coupled to the source of the n-channel pass-gate transistor of the pair of n-channel pass-gate transistors. A word line is coupled to the gates of the n-channel pass-gate transistors.

Aspects of the present invention also provide a method of forming an SRAM cell. A silicon-on-insulator substrate includes a silicon layer overlying an insulator layer, e.g., an oxide. At least one active region is defined in the silicon layer. A gate dielectric layer is formed over the active region and a number of gate electrodes are formed overlying the gate dielectric layer. Source and drain regions are formed adjacent to the plurality of gate electrodes to form a fully depleted transistor and a partially depleted transistor.

Advantages of the preferred embodiment of this invention include providing a structure and method for the integration of fully-depleted and partially-depleted SOI transistors, as well as multiple-gate transistors. Aspects of this invention describes an SRAM cell structure that employs both FD-SOI and PD-SOI transistors; improved performance of SOI SRAM cell configured using FD-SOI and PD-SOI transistors; use of FD-SOI transistors as pass-gate transistors or access transistors eliminate problems associated with the transient bipolar effect; and use of FD-SOI transistors as pull-up transistors to eliminate problems associated with history effect.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a schematic diagram of an SRAM that can utilize aspects of the present invention;

FIG. 2 is a schematic diagram of an SRAM array;

FIG. 3 is a cross-sectional diagram of a first embodiment;

FIGS. 4 a-4 d illustrate partially depleted SOI transistors and a fully depleted SOI transistors of the present invention;

FIGS. 5 a and 5 b each provide a map showing the region of PD-SOI, FD-SOI, and multiple-gate transistors as a function of width W and length L_(g) for NMOS (FIG. 5 a) and PMOS (FIG. 5 b) transistors;

FIG. 6 shows a layout of an embodiment SRAM cell of the present invention; and

FIGS. 7 a-7 c and 8 a-8 b provide perspective views of a device during manufacture.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

The present invention is related to U.S. Pat. No. 6,855,990 B2 entitled “Strained-Channel Multiple-Gate Transistor”, which issued Feb. 15, 2005, U.S. Pat. No. 6,720,619 B1 entitled “Semiconductor-on-Insulator Chip Incorporating Partially-Depleted, Fully-Depleted, and Multiple-Gate Devices”, which issued Apr. 13, 2004, and U.S. Pat. No. 6,867,433 B2 entitled “Semiconductor-on-Insulator Chip Incorporating Strained-Channel Partially-Depleted, Fully-Depleted, and Multiple-Gate Transistors,” which issued Mar. 15, 2005. Each of these applications are incorporated herein by reference. Aspects of the present invention build upon and provide improvements and applications of previous inventions described in the incorporated applications.

In one aspect, the present invention relates to static random access memory (SRAM). SRAM is commonly used in data processors for the storage of instructions and data. SRAM is generally used in data processors requiring optimally fast access rates. Recent SRAM designs have exploited the advantages of PD-SOI technology. By making SRAM on SOI substrates, performance enhancement of up to 20-30% may be achieved compared to SRAM made on bulk silicon (Si) substrates.

FIG. 1 shows a schematic diagram of a 6T (six transistor) CMOS SRAM cell 100. Such an SRAM cell 100 typically includes six metal-oxide-semiconductor field effect transistors (MOSFETs): two P-channel field-effect transistors (PFETs) 102 and 104 for a pull-up operation, two N-channel field-effect transistors (NFET) 106 and 108 for pull-down, and two NFETs 110 and 112 for input/output access, i.e. pass-gate access. As shown in FIG. 1, P1 and N1 form an inverter, which is cross-coupled with another inverter that includes P2 and N2. The devices 110 and 112 are pass-gate access devices that control reading from and writing into the SRAM cell 100.

The left bit line (BL) 114, the right bit line (BR) 116, the ground line (GND) 118 and supply node (VDD) 122 are also shown. The left bit line 114 carries a signal that is complementary to the signal carried on right bit line 116. As a result, the complementary bit line pair 114 and 116 (or 116 and 114) is sometimes referred to as bit line and bit line bar. A word line (WL) 120 is coupled to the gates of the pass transistors 110 and 112 so that a logic value stored in the memory cell 100 can be placed on the bit lines 114 and 116 or vice versa.

A typical SRAM array includes a matrix of m rows and n columns of the aforementioned SRAM cells 100 as shown in FIG. 2. Cells of the same row share one WL 120 (e.g., WL₀ or WL₁), while cells of the same column share the same complementary bit line pair 114 and 116 (e.g., BL₀ and BL ₀ or BL₁ and BL ₁). The aforementioned design is used in many SRAMs, including for example, a 1-mega-bit memory having typically 1024 by 1024 cells 100.

In a prior art SRAM cell configured using PD-SOI transistors, all the transistors in the cell are partially depleted SOI transistors. SRAM cells configured using PD-SOI transistors, however, suffer from problems related to the floating body effect. A floating body region may be charged up when the source and drain of a transistor are biased at the supply voltage.

For example, referring to FIG. 2, assume that WL₀ is selected (e.g., at a high voltage level) and WL₁ is unselected (e.g., at a low voltage level). The pass-gate transistor 110 of the unselected cell 100 c may be an n-channel transistor having the source and drain regions at an initial high potential when a physical “1” is stored in the cell. The p-type body region will be charged up to the same potential level as the source and drain regions. When left bit line BL₀ of the unselected cell 100 c is suddenly put to ground bias (GND), the source region of the pass-gate transistor is suddenly grounded and the body-source p-n junction will be turned on. The charge accumulated in the floating body will flow out of the transistor, contributing to a parasitic current flow from the transistor into bit line BL₀. The current is also known as a parasitic bipolar leakage current. This may degrade the noise margin and stability of the SRAM circuit.

In certain dynamic circuits, the parasitic bipolar effect causes logic state errors if not properly accounted for. In an SRAM array, the parasitic bipolar leakage current contributed by the pass-gate transistors 110 or 112 causes the SRAM array to be up to 20% slower than if the leakage current was non-existent. This is known as the transient bipolar effect and has been reported by Kuang et al. in IEEE Journal of Solid-State Circuits, vol. 32, no. 6, pages 837-844, June 1997, which paper is incorporated herein by reference.

In one aspect, the present invention provides a method and system to overcome the shortcomings of the prior art, and to provide a highly manufacturable PD-SOI-like technology that produces FD-SOI type devices to eliminate floating body effects, e.g. history effect. A silicon-on-insulator technology has been successfully developed that integrates PD-SOI and FD-SOI transistors on the same chip. In this patent, the abovementioned SOI technology can be used to form a static random access memory (SRAM) cell that employs both PD-SOI and FD-SOI transistors.

A first embodiment of this invention provides an SRAM cell with both PD-SOI and FD-SOI transistors. Another embodiment of this invention teaches a method of forming such an SRAM cell. By incorporating FD-SOI transistors in critical parts of the SRAM cell that is susceptible to undesirable floating body effects, performance of an SRAM array may be significantly improved. In addition, a structure and method of forming an SRAM cell with FD-SOI and PD-SOI transistors with enhanced performance is provided.

The preferred embodiment of this invention employs an SOI technology that incorporates PD-SOI and FD-SOI transistors in the same manufacturing process, i.e., PD-SOI transistors and FD-SOI transistors may be formed on the same semiconductor substrate and these transistors may be formed in close proximity to each other. Therefore, by using the SOI technology of this invention, a SRAM cell may be designed such that both PD-SOI and FD-SOI transistors may be used. Embodiments of this invention may also employ the option of introducing strain in the channel of these SOI transistors to enhance the performance of the SRAM cell.

The schematic of an SRAM cell according to one embodiment of this invention is depicted in FIG. 1. As discussed above, the schematic diagram of FIG. 1 shows a six-transistor static random access memory (SRAM) cell 100. In the preferred embodiment, both PD-SOI and FD-SOI transistors are incorporated in the same SRAM cell 100.

For example, in one embodiment the four transistors 102, 104, 106 and 108 constituting the two cross-couple inverters are formed from partially depleted SOI transistors. As before, the PD-SOI transistors 102 and 106 constitute the first inverter and the PD-SOI transistors 104 and 108 constitute the second inverter. The two pass-gate transistors 110 and 112, sometimes also known as access transistors, are fully depleted SOI transistors. In the preferred embodiment, the transistors 110 and 112 are both n-channel transistors, although they may be p-channel transistors.

By using FD-SOI transistors as pass-gate or access transistors 110 and 112, the problem associated with the bipolar leakage current or the transient bipolar effect can be avoided. Floating body effects are absent in FD-SOI transistors. FD-SOI transistors do not suffer from floating body effects due to the fact that the body is fully depleted, and no portion of the body region is undepleted. An SOI transistor may have a fully depleted body if it has a low body doping and/or a thin body thickness. Additionally, for good control of short channel effects in ultra-scaled devices, the device body thickness may be reduced to less than one third of the gate length. Such a thin body thickness may require raised source/drain technology for series resistance reduction.

According to another embodiment of this invention, the pull-up transistors 102 and 104 are FD-SOI transistors. In this embodiment, the pass-gate transistors 110 and 112 may be PD-SOI transistors but they are preferably FD-SOI transistors. The pull-down transistors 106 and 108 are preferably PD-SOI transistors but could be FD-SOI transistors. When the pull-down transistors 106 and 108 are FD-SOI transistors, the floating-body effects and threshold voltage instability problems associated with these transistors will be eliminated, leading to a larger window and noise margin in the operation of the memory cell. This will be more important if the supply voltage is reduced.

To this point, the SRAM cells have been described in the context of a memory array, i.e., a two-dimensional matrix of rows and columns of memory cells. The concepts, however, would apply equally to other devices. For example, many latches, registers, FIFOs and other devices include cross-coupled inverters such as those shown in FIGS. 1 and 2. Any of these elements could benefit from aspects of the present invention.

FIG. 3 shows a cross-sectional view of a device that includes a partially depleted SOI transistor 130 and two fully depleted SOI transistors 132. These transistors 130 and 132 are formed on an SOI substrate that includes a substrate 134 (e.g., silicon) and an overlying insulating layer 136 (e.g., a buried oxide). The PD-SOI 130 is formed in silicon layer 138 and FD-SOI transistors 132 are formed in silicon layer 140. Isolation regions 142 isolate separate active areas.

The SRAM cell 100 of FIG. 1 can be implemented using structures as shown in FIG. 3. For example, the FD-SOI transistors in layer 140 mentioned in the first and second structure SRAM embodiments can be implemented where the body thickness is thinner than the maximum depletion width W_(d,max). The depletion width W_(d,max) is given by the equation:

$\begin{matrix} {W_{d,\max} = \sqrt{\frac{4ɛ_{s}\phi_{b}}{{qN}_{a}}}} & (1) \end{matrix}$ where ε_(s) is the permittivity of the body region of the transistor, q is the electronic charge, N_(a) is the average doping concentration in the body region of the transistor, and φ_(b) is given by

$\begin{matrix} {\phi_{b} = {\frac{kT}{q}{\ln\left( \frac{N_{a}}{n_{i}} \right)}}} & (2) \end{matrix}$ where k is Boltzmann's constant, T is temperature, and n_(i) is the intrinsic carrier concentration. For silicon, n_(i) is 1.45×10¹⁰ cm³.

The doping concentration N_(a) may be determined indirectly from the known threshold voltage of the transistor using the equation:

$\begin{matrix} {V_{th} = {\left( {\Phi_{M} - \Phi_{S}} \right) + {2\phi_{b}} + \sqrt{\frac{4ɛ_{s}{qN}_{a}\phi_{b}}{ɛ_{d}/t_{d}}}}} & (3) \end{matrix}$ where t_(d) is the physical thickness of the gate dielectric, ε_(d) is the permittivity of the gate dielectric, and Φ_(M) is the workfunction of the gate electrode material, and Φ_(S) is the workfunction of the material constituting the channel region of the transistor.

Since threshold voltage V_(th) of transistor may be known or easily determined, and the value of (Φ_(M)−Φ_(S)) is known if the gate electrode material is known, the V_(th) equation given above may be solved for N_(a). N_(a) may be determined from V_(th), as described, or determined from other known physical or experimental analysis techniques.

The value of N_(a) may then be used to compute the maximum depletion width. If the computed W_(d,max) is larger than the thickness of the body region, the transistor is a FD-SOI transistor. If the computed W_(d,max) is smaller than the thickness of the body region, the transistor is a PD-SOI transistor. This is a first way of determining whether a SOI transistor is a FD-SOI transistor or a PD-SOI transistor.

A second way is similar to the first way, by expressing W_(d,max) in terms of V_(th). Rearranging equation (3):

$\begin{matrix} {\sqrt{\left( {4ɛ_{s}{qN}_{a}\phi_{b}} \right)} = {\left\lbrack {V_{th} - \left( {\Phi_{M} - \Phi_{S}} \right) - {2\phi_{b}}} \right\rbrack\left( {t_{d}/ɛ_{d}} \right)}} & (4) \end{matrix}$ Taking reciprocal on both sides and multiplying by a factor 4ε_(S)φ_(b), we obtain

$\begin{matrix} {\frac{4ɛ_{s}\phi_{b}}{\sqrt{N\; ɛ_{s}{qN}_{a}\phi_{b}}} = \frac{4ɛ_{s}\phi_{b}}{\left( {V_{tb} - \left( {\Phi_{M} - \Phi_{S}} \right) - {2\phi_{b}}} \right) \cdot \left( {t_{d}/ɛ_{d}} \right)}} & (5) \end{matrix}$ The left-hand side of equation (5) is W_(d,max), giving

$\begin{matrix} {W_{d,\max} = {\frac{4ɛ_{s}\phi_{b}}{\sqrt{N\; ɛ_{s}q\; N_{a}\phi_{b}}} = \frac{4ɛ_{s}\phi_{b}}{\left( {V_{tb} - \left( {\Phi_{M} - \Phi_{S}} \right) - {2\phi_{b}}} \right) \cdot \left( {t_{d}/ɛ_{d}} \right)}}} & (6) \end{matrix}$

Since Φ_(S) for an n-channel transistor is (4.61+φ_(b)) and Φ_(S) for a p-channel transistor is (4.61−φ_(b)), we have:

$\begin{matrix} {{{for}\mspace{14mu} n\text{-}{channel}\mspace{14mu}{transistor}\mspace{14mu} W_{d,\max}} = \sqrt{\frac{4ɛ_{s}\phi_{b}}{\left\{ {\left\lbrack {V_{th} - \Phi_{M} + 4.61 - \phi_{b}} \right\rbrack\left( {t_{d}/ɛ_{d}} \right)} \right\}}}} & \left( {7n} \right) \\ {{{for}\mspace{14mu} p\text{-}{channel}\mspace{14mu}{transistor}\mspace{14mu} W_{d,\max}} = \sqrt{\frac{4ɛ_{s}\phi_{b}}{\left\{ {\left\lbrack {V_{th} - \Phi_{M} + 4.61 - {3\phi_{b}}} \right\rbrack\left( {t_{d}/ɛ_{d}} \right)} \right\}}}} & \left( {7p} \right) \end{matrix}$

A third way is to examine the drain current versus drain voltage (I_(DS)−V_(DS)) characteristics of the SOI transistor. If kinks or discontinuities in the slope of the I_(DS)−V_(DS) characteristics exist, the transistor is a PD-SOI transistor. Otherwise, the transistor is a FD-SOI transistor.

In FIG. 3, PD-SOI and FD-SOI transistors are formed on the same substrate by modifying the thickness, using silicon thickness larger than W_(d,max) for PD-SOI transistors 130 and using silicon thickness smaller than W_(d,max) for FD-SOI transistors 132. In another embodiment, some of the transistors are formed in a silicon layer 140 that is thinner than silicon layer 138, regardless of whether these transistors are FD-SOI or PD-SOI transistors.

In the preferred embodiment, however, the FD-SOI transistors are not formed by modifying the silicon thickness. In the preferred embodiment, the FD-SOI transistors are three-dimensional or FinFET-like FD-SOI transistors, which makes use of a novel device geometry to eliminate floating body effects. In general, planar FD-SOI transistors have widths of more than 50 nm while non-planar fully depleted multiple-gate transistors have widths of less than 50 nm.

Concepts of the formation of both PD-SOI transistors and FD-SOI transistors are more clearly illustrated in FIGS. 4 a-4 d. FIGS. 4 a-4 d illustrate PD-SOI transistors and FD-SOI transistors described in U.S. Pat. No. 6,720,619 B1 entitled “Semiconductor-on-Insulator Chip Incorporating Partially-Depleted, Fully-Depleted, and Multiple-Gate Devices”, which issued Apr. 13, 2004. The FD-SOI transistor of FIG. 4 b uses a low body doping so that the maximum depletion width is larger than the silicon thickness to achieve full depletion. The FD-SOI transistor of FIG. 4 d uses a novel geometry to allow the encroachment of gate electric field from the sides of the silicon body to achieve full body depletion.

Super-halo doping and light body doping are designed to achieve FD-SOI and PD-SOI devices at different gate lengths, as shown in FIGS. 4 a and 4 b. Referring first to FIG. 4 a, a partially depleted transistor device 150 is formed over a buried insulator 160. While not shown, the buried insulator 160 is formed over a substrate, e.g., an undoped or lightly doped silicon substrate (see substrate 134 in FIG. 3).

The buried insulator 160 is typically an oxide such as silicon dioxide. Other insulators, such as silicon nitride or aluminum oxide, may alternatively be used. In some embodiments, the buried insulator can comprise a stack of layers, e.g., an oxide, nitride, oxide stack.

Transistor device 150 is formed in a semiconductor layer 162 and includes a source region 164 and a drain region 166. A gate 168 overlies a channel 170 and is separated therefrom by gate dielectric 172.

Similarly long-channel transistor 180 includes a source 182, a drain 184, a gate 186, and a gate dielectric 188. The transistor 180 can be formed in the same semiconductor layer 162 as transistor 150 or in a different semiconductor layer e.g., a different island or mesa on the same chips.

One feature is the design of the super-halo doping 190 in FIG. 4 b (or double halo doping 190 in FIG. 4 a) and light body doping 192 such that the effective doping concentration of the transistor body decreases as the gate length is increased. The doping concentration in the super-halo region 190 is in the range of about 1×10¹⁸ to about 2×10¹⁹ dopants per cubic centimeter. The doping concentration in the lightly doped body region 192 is in the range of about 1×10¹⁶ to about 1×10¹⁸ dopants per cubic centimeter.

In FIG. 4 a, the high super-halo doping concentration 190 in a short-channel transistor 150 results in a maximum depletion width that is smaller than the silicon film 162 thickness, and the transistor body is therefore partially-depleted. As the gate 168 length increases, an increasing portion of the body region is constituted by the lightly doped body region 192, and the average effective body concentration decreases. Consequently, the maximum depletion width increases with an increase in the transistor gate length or channel length. In FIG. 4 b, the long-channel transistor 180 has a light body doping and a maximum depletion width that is larger than the silicon film 162 thickness, and the transistor body is fully depleted.

Referring now to FIGS. 4 c and 4 d, another way to achieve full depletion in the transistor body is to allow the electric field lines to encroach from the sides of the transistor body by using a novel transistor geometry. Referring now to FIG. 4 c, a transistor 200 is formed over a buried insulator. The buried insulator 202 can include any of the characteristics described above with respect to insulator 160 and may be formed on a substrate, where the discussion above with respect to FIGS. 4 a and 4 b equally applies here. In this device, an active semiconductor layer region 205 includes a body region 204 and a depletion region 206. The active region 205 is isolated from the other active regions by isolation region 208. This isolation region 208 is preferably a shallow trench isolation (STI) region. It is understood that other isolation structures may be used.

A gate electrode 210 is formed to surround the transistor active region, e.g., the channel region. Accordingly, an intentional recess 212 is formed within the isolation region 208 so that the semiconductor layer 205 includes sidewalls. The gate electrode 210 is adjacent to the top surface as well as the sidewalls of active layer 205. A gate dielectric layer 214 is formed between the gate electrode 210 and the active layer 205.

The source and drain regions of the transistor device 200 are not shown in the illustration of FIG. 4 c. In this case, the channel current flows into and out of the page. As a result, one of the source/drain regions will be located in a plane above the page and the other located in a plane below the page.

FIG. 4 d shows a similar structure for a FinFET-like transistor device 220. Like elements from FIG. 4 c have been labeled with the same reference numerals. In this case, the active semiconductor layer is thin casing the body to be fully depleted.

One feature of the novel transistor geometry is the intentional recess 212 in the isolation region 208, as shown in FIGS. 4 c and 4 d. The planar partially depleted transistor 200 of FIG. 4 c has a width that is much bigger than the maximum depletion layer width W_(d,max). When the active region width W (see FIG. 4 d) is reduced to less than twice the depletion width layer in the body, the gate field encroaches from the isolation edges and eliminates the undepleted body region, thereby making the device of FIG. 4 d fully depleted.

The resulting FD-SOI device has a non-planar geometry and is a multiple-gate transistor where the gate electrode 210 surrounds the transistor body 206 on multiple sides: the two sidewalls and the top surface. By having a gate electrode 210 that surrounds the transistor body 205, the multiple-gate transistor allows the encroachment of the gate electric field to the transistor body in the lateral direction, thus enhancing its ability to control short-channel effects.

The preferred embodiment of this invention teaches a unique way of incorporating PD-SOI and FD-SOI transistors on the same chip using the same process technology, with a distribution of FD-SOI and PD-SOI transistors according to transistor dimensions. FIGS. 5 a and 5 b (collectively FIG. 5) show the distribution of the PD-SOI and FD-SOI transistors according to the active region width W and the transistor gate length L_(g). FIG. 5 a provides data for NMOS devices and FIG. 5 b shows data for PMOS devices. These figures provide a map showing the region of PD-SOI transistors (gray region), conventional FD-SOI transistors (white region), and multiple-gate transistors (in region enclosed by dashed box) as a function of width W and length L_(g) for NMOS and PMOS transistors.

Planar PD-SOI and FD-SOI transistors typically have active region width of more than 50 nm, while non-planar multiple-gate fully depleted transistors generally have active region width of less than 50 nm. The results in FIG. 5 are obtained from an experiment where transistors are fabricated using a 65 nm PD-SOI-based process with a nominal gate length of 45 nm, a silicon body thickness of 40 nm, dual-doped poly-silicon gate electrodes, 14 angstroms nitrided gate oxide, and cobalt-silicided source/drain and gate.

The PD-SOI region is smaller for P-channel transistors (FIG. 5 b) than for N-channel transistors (FIG. 5 a) because the impact ionization induced parasitic bipolar action is weaker in P-channel transistors. The transition from PD- to FD-SOI occurs as the gate length is increased. In addition, the non-planar FinFET-like or multiple-gate transistors are obtained at small width W, typically less than 50 nm. Wide-channel devices with smaller gate length L_(g) are partially depleted, showing a characteristic kink in the drain current I_(DS) versus drain voltage V_(DS) curves. As W is reduced, transition from PD-SOI to FD-SOI occurs and the characteristic I_(DS)−V_(DS) kink disappears.

It is clear that the advantages of PD-SOI and FD-SOI can be combined by using transistors with different combinations of W and L_(g). For example, when converting a circuit design for bulk technology to a circuit design for SOI technology, critical portions of the circuits may employ FD-SOI devices to achieve minimal floating body effects while the remaining portions of the circuits may employ PD-SOI devices. For example, the critical portions of the circuits may include analog circuits and dynamic circuits.

An example of the layout implementation of an SRAM cell is illustrated in FIG. 6. It is understood that other layout implementations can be used. For the sake of simplicity, metal layout shapes are not shown. In this particular layout, the word line (WL) 120 is shown along the horizontal direction. The left bit line 114, the right bit line 116, and the ground line GND 118 are also shown. The dimensions of the transistors are also labeled. The width and length of each transistor are labeled W and L respectively, and the name of the transistor is indicated in the subscript. For example, W_(PG1,FD) and L_(PG1,FD) label the width and length of the FD-SOI pass-gate transistor labeled 102.

According to the preferred embodiment of this invention, the width and length of the pass-gate transistors 102 and 104 are selected such that they lie in the FD-SOI or multiple-gate transistor regions (white regions) in FIG. 5 a if they are n-channel transistors or in the FD-SOI or multiple-gate transistor regions (white regions) in FIG. 5 b if they are p-channel transistors. More preferably, the width and length of the pass-gate transistors 102 and 104 are selected such that they lie in the multiple-gate FinFET-like transistor regions. Such transistors typically have layout widths that are about 50 nm or smaller for n-channel transistor and about 60 nm or smaller for p-channel transistor. The gate length for transistors 102 and 104 are selected such that they are typically larger than those of the pull-down transistors 106 and 108. In the preferred embodiment, the pull-down transistors 106 and 108 are PD-SOI transistors.

The ratio of the conductance of pull-down transistor 106 (108) over the conductance of pass-transistor 110 (112) can be used as a basic metric to measure the stability of the SRAM cell or the ability of the cell to retain its data state. This ratio is referred to by CMOS SRAM designers as ‘beta’ or ‘beta ratio’. It is defined as the ratio of the conductance of the pull-down device over the conductance of pass-gate device. The larger the beta ratio, the cell becomes more stable, and its static noise margin (SNM) increases. The conductance of a transistor is approximately proportional to the effective carrier mobility μ_(eff) and the ratio of the device width to the channel length, i.e., W/L. Accordingly, the beta of the SRAM cell can be approximated by the ratio of μ_(eff)(W/L) of transistor 106 and μ_(eff)(W/L) of transistor 110. If transistors 106 and 110 have the same channel length, then the beta ratio becomes the ratio of the channel width of transistor 106 over the channel width of transistor 110. Depending on the SRAM application, the beta preferably ranges from about 1.8 to about 3.

An SRAM cell of the present invention may additionally employ transistors with strained channel regions. For example, the transistors constituting the SRAM cell of FIG. 1 may all be strained channel transistors. The use of strained channel transistors is expected to enhance the performance of the SRAM cell significantly. According, transistors 110 and 112 may be strained channel FD-SOI transistors, and transistors 102, 104, 106 and 108 may be strained channel PD-SOI transistors. Use of appropriate strain enhances carrier mobility and strain-induced mobility enhancement is another way to improve transistor performance in addition to device scaling.

The incorporation of strain in the channel region of PD-SOI and FD-SOI transistors on the same chip is described in U.S. Pat. No. 6,867,433 B2 entitled “Semiconductor-on-Insulator Chip Incorporating Strained-Channel Partially-Depleted, Fully-Depleted, and Multiple-Gate Transistors,” which issued Mar. 15, 2005, and incorporated herein by reference. In this approach, a high stress film is formed over a completed transistor structure. The stressor, i.e. the high stress film, exerts significant influence on the channel, modifying the silicon lattice spacing in the channel region, and thus introducing strain in the channel region.

A method of fabricating the above-mentioned SRAM cell with partially depleted SOI transistors and fully depleted SOI transistors or multiple-gate transistors will be described next with respect to FIGS. 7 a-7 c. The starting material is a semiconductor-on-insulator wafer 250. The semiconductor-on-insulator wafer comprises of a semiconductor layer 252 overlying an insulator layer 254, which in turn overlies a substrate 256, as shown in FIG. 7 a. The semiconductor layer 252 can be an elemental semiconductor such as silicon and germanium, an alloy semiconductor such as silicon-germanium, or a compound semiconductor such as gallium arsenide and indium phosphide. In the preferred embodiment, the semiconductor layer 252 is silicon, preferably monocrystalline silicon.

The insulator layer 254 may be any insulating material such as silicon oxide, aluminum oxide, or silicon nitride. In the preferred embodiment, insulating material 254 is silicon oxide, preferably SiO₂. This layer may be deposited on substrate 256 prior to forming semiconductor layer 252. Alternatively, a SIMOX process can be used to implant oxygen into a substrate that initially includes both layers 252 and 256. The underlying substrate 256 may be any semiconductor substrate such as silicon substrate or gallium arsenide substrate. In other embodiments, other substrates such as ceramic or quartz can be used.

In the preferred embodiment, the semiconductor layer 252 is silicon and the insulator 254 is silicon oxide. More preferably, the silicon layer 252 in the preferred embodiment has a thickness in the range of about 10 angstroms to about 2000 angstroms and the silicon oxide layer has a thickness in the range of about 100 to about 2000 angstroms.

Referring now to FIG. 7 b, an active region or silicon fin 258 is formed by patterning the silicon layer 252. The patterning of the active region or silicon fin may be accomplished, for example, by depositing a mask material (not shown) on the silicon layer 252, patterning the mask material by optical lithography to form a patterned mask (not shown), etching the silicon layer 252, and removing the patterned mask. The mask material can be a photoresist, silicon nitride, or a stack comprising of a silicon nitride layer overlying a silicon oxide layer. Isolation regions 260, preferably comprising a dielectric such as silicon oxide are formed to isolate silicon active region 258 from other active regions (e.g., fins) that are not shown in the figure.

Referring now to FIG. 7 c, a gate dielectric is formed. The gate dielectric layer 262 can have a thickness between about 3 to about 100 angstroms. The gate dielectric layer 262 on top of the active region 258 can have a different thickness than the gate dielectric layer 262 on the sidewalls of the active area 258. For example, the thickness of the gate dielectric layer 262 on the top can be thinner than that on the sidewall. In some examples, the thickness of the gate dielectric layer 262 on top of the active region 258 is less than about 20 angstroms.

The gate dielectric may comprise a gate dielectric material such as silicon oxide, silicon oxynitride, or nitrided silicon oxide or combinations thereof. The insulating material may also be a high permittivity material with permittivity larger than 5, such as aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), hafnium silicon oxynitride (HfSiON), hafnium silicate (HfSiO₄), zirconium oxide (ZrO₂), zirconium silicate (ZrSiO₄), lanthanum oxide (La₂O₃).

In the preferred embodiment, the gate dielectric 262 is silicon oxide, which may be formed by thermal oxidation in an oxygen ambient at temperatures ranging from about 500 to about 1000 degrees Celsius. The gate dielectric layer 262 can alternatively be formed by chemical vapor deposition or reactive sputtering. The gate dielectric layer 262 covers the top and the sidewalls of the silicon fin 258.

With the gate dielectric layer 262 appropriately formed, gate electrode material 264 can then be formed on top of the gate dielectric layer 262. The gate electrode material 264 can be formed from poly-crystalline silicon, poly-crystalline silicon germanium, metals, metallic silicides, metallic nitrides, or metallic oxides. For example, metals such as molybdenum, tungsten, titanium, tantalum, platinum, and hafnium may be used as the portion of the gate electrode 264. Metallic silicides may include, but will not be limited to, nickel silicide, cobalt silicide, tungsten silicide, titanium silicide, tantalum silicide, platinum silicide, and erbium silicide. Metallic nitrides may include, but will not be limited to, molybdenum nitride, tungsten nitride, titanium nitride, and tantalum nitride. Metallic oxides may include, but will not be limited to, ruthenium oxide and indium tin oxide.

The gate electrode material 264 can be deposited by conventional techniques such as chemical vapor deposition. For example, the gate electrode can be formed by the deposition of silicon and a refractory metal, followed by an anneal to form a metal silicide gate electrode material. In embodiments, the refractory metal can be titanium, tantalum, cobalt, or nickel.

The gate electrode material 264 is then patterned using photolithography techniques, and etched using plasma etch processes to form the gate electrodes. FIG. 7 c shows the gate electrodes of the FD-SOI pass-gate transistor PG1 _(FD) and the PD-SOI pull-down transistor N1 _(FD) after gate electrode formation (see FIG. 6). The gate length and width of the transistors are also indicated in the three-dimensional figure shown in FIG. 7 c. The gate dielectric 262 is retained at least in the portion of the device covered by the gate electrode 264.

Additional process steps will now be described with respect to FIGS. 8 a and 8 b, which show only one of the transistors.

The source 266 and drain 268 extensions (e.g., lightly doped source/drain) are then implanted using ion implantation techniques. Super-halo implant may also be performed at this stage. By using implanting the super-halo implant at a large angle ranging from 15 to 45 degrees with respect to the normal of the wafer, devices with short channel lengths will receive a high effective channel doping concentration, while devices with long channel lengths will receive a low effective channel doping concentration.

A spacer 270 is formed using techniques known and used in the art, e.g. deposition of the spacer material and anisotropic plasma etching. The spacer material may comprise of a dielectric material such as silicon nitride or silicon dioxide. In the preferred embodiment, the spacer 270 is silicon nitride.

Following spacer formation, the source and drain regions 266 and 268 are implanted. The source and drain regions can be strapped with one or more conductive materials such as metals and silicides (not shown). The conductive materials can reach the source and drain regions through contacts on the sidewalls and/or the top of the active region.

Next, a high-stress film or a stressor 272 is deposited over the completed transistor structure as shown in FIG. 8 b. According to this invention, the high stress film contacts not only the top surface but also the sidewall surfaces of the active region 258. As an example, the high-stress film 272 can be PECVD silicon nitride. PECVD silicon nitride can be used to introduce tensile or compressive stress in the channel region. The residual film stress impacts the strain components in the channel. The residual film stress can be tailored from a high state of tension, for stoichiometric silicon nitride, to one of compression, for silicon-rich films. The tensile or compressive nature of the strain in the channel region can therefore be adjusted by varying process conditions such as temperature, pressure, and the ratio of the flow rate of a precursor gas, e.g. dichlorosilane, to the total gas flow rate.

Following the formation of the high-stress film 272, a passivation layer (not shown) is deposited with a thickness of a few thousand angstroms, e.g. 1000 to 5000 angstroms. The passivation layer is preferably comprised of silicon oxide. Contact holes (not shown) are etched through the passivation layer and the high-stress film 272. Conductive materials (not shown) are then used to fill the contact holes to electrically contact the source region 266, drain region 268, and gate electrode 264 of the transistor.

Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

1. A method of forming a SRAM cell, the method comprising: providing a silicon-on-insulator substrate, said substrate comprising a silicon layer overlying an insulator layer; defining at least one active region in the silicon layer; forming a gate dielectric layer in the active region; forming a plurality of gate electrodes overlying the gate dielectric layer; and forming source and drain regions adjacent to the plurality of gate electrodes to form a fully depleted transistor and a partially depleted transistor as two transistors of the SRAM cell, wherein the fully depleted transistor has a greater channel length than the partially depleted transistor.
 2. The method of claim 1 wherein the fully depleted transistor is a pass-gate transistor of the SRAM cell.
 3. The method of claim 1 wherein the partially depleted transistor is a pull-down or a pull-up transistor of the SRAM cell.
 4. The method of claim 1 further comprising a step of forming a high-stress film covering the gate electrode, source region, and drain region of at least one of the fully depleted transistor or the partially depleted transistor.
 5. The method of claim 4 wherein the high stress film contacts a sidewall surface of the active region.
 6. The method of claim 1 and further comprising, after forming the gate electrode, forming source and drain extension regions.
 7. The method of claim 6 and further comprising, after forming the source and drain extension regions, forming super halo region.
 8. The method of claim 7 wherein the super halo region is formed by ion implantation.
 9. The method of claim 8 wherein the super halo region has a doping concentration in the range of about 1×10¹⁸ to about 2×10¹⁹ cm⁻³.
 10. The method of claim 1 wherein the silicon layer has a thickness in the range of about 10 angstroms to about 2000 angstroms.
 11. The method of claim 1 wherein the silicon layer has a thickness of less than about 200 angstroms.
 12. The method of claim 1 wherein the active regions have rounded corners.
 13. The method of claim 12 wherein at least one of the rounded corners has a radius of about 10 angstroms to about 200 angstroms.
 14. A method of forming a SRAM cell, the method comprising: providing a silicon-on-insulator substrate, said substrate comprising a silicon layer overlying an insulator layer; forming one or more active regions within the substrate, at least one of the one or more active regions having a first section with a first width and a second section with a second width, the first width being different than the second width; forming a fully depleted pass-gate transistor by a method comprising: forming a gate dielectric layer over the one or more active regions; forming a gate electrode layer over the gate dielectric layer; patterning the gate electrode layer to form a first gate electrode in the first section of the at least one of the one or more active regions, wherein the first gate electrode has a first gate length; and forming first source and drain regions of opposing sides of the first gate electrode; and forming a partially depleted transistor by a method comprising: patterning the gate electrode layer to form a second gate electrode in the second section of the at least one of the one or more active regions, wherein the second gate electrode has a second gate length smaller than the first gate length; and forming second source and drain regions on opposing sides of the second gate electrode.
 15. The method of claim 14 wherein the partially depleted transistor is a pull-down or a pull-up transistor of the SRAM cell.
 16. The method of claim 14, further comprising: after forming the first gate electrode, forming first source and drain extension regions in the first section and a first super halo region; and after forming the second gate electrode, forming second source and drain extension regions in the second section and a second super halo region.
 17. A method for forming an SRAM memory array, the method comprising: providing a substrate, said substrate comprising a semiconductor layer overlying an insulator layer; defining a plurality of active regions in the silicon layer; forming a plurality of SRAM memory cells, each cell formed by: forming a gate dielectric layer in at least one of the plurality of active regions; forming a plurality of gate electrodes overlying the gate dielectric layer; and forming first source and drain regions adjacent to respective ones of the plurality of gate electrodes to form a fully depleted FinFET transistor, the fully depleted FinFET transistor having a first channel length; forming second source and drain regions adjacent to respective ones of the plurality of gate electrodes to form a partially depleted transistor, the partially depleted transistor having a second channel length that is less than the first channel length; forming a plurality of complementary bit line pairs connected to respective ones of the plurality of SRAM memory cells; and forming a plurality of word lines connected to respective ones of the plurality of SRAM memory cells.
 18. The method of claim 17 wherein the fully depleted transistor is a pass-gate transistor of one of the plurality of SRAM memory cells.
 19. The method of claim 17 wherein the partially depleted transistor is a pull-down or a pull-up transistor of one of the plurality of SRAM memory cells.
 20. The method of claim 17 further comprising a step of forming a high-stress film covering the gate electrode, source region, and drain region of at least one of the fully depleted transistor or the partially depleted transistor. 